1. Field of the Invention
The invention generally relates to electronics, and in particular, to frequency division duplex radios.
2. Description of the Related Art
Echo cancellation is a well-known technique in transceiver design. The application of echo cancellation within the RF domain is less well known.
In a frequency division duplexing (FDD) radio, the channels for transmission and reception are separated by a difference in carrier frequency. FIG. 1 illustrates a frequency spectrum with an Uplink and a Downlink, or Tx and Rx, respectively, which can beneficially be transmitted simultaneously (full duplex). Frequency division duplexing is used in both wired and wireless systems. For example, FDD is found in DSL modems, cell phone systems, IEEE 802.16 WiMax systems, and the like.
In a wireless system, when two transceivers in a communications link are spaced far apart, the transmitted (Tx) signal of a transceiver will have much more power than the received (Rx) signal at the antenna, as illustrated by FIG. 2 (not to scale).
Moreover, in a typical wireless system, the transmitter and the receiver of a transceiver share a common antenna. To share the antenna, a duplexer is introduced into the transceiver front end to attenuate the transmit signal (seen at the input of the Rx), as shown in FIGS. 3 and 4.
Typical duplexers offer about 50 dB of attenuation to the Tx signal. This means that the residual Tx signal or echo signal (seen at the Rx (or LNA) input) can still be quite large at the input to the LNA.
To tolerate this relatively large echo signal, the linearity, expressed as intercept points IP2 and IP3 of the LNA, mixer, and analog baseband should be increased. An increase in the linearity of a wireless transceiver results in increases in size, power, and cost.
DSP-based echo cancellation is a well known technique for wired transceiver design, such as, for example, with Gigabit Ethernet over copper. However, DSP echo cancellation methods are inapplicable to a wireless transceiver because the nonlinearity of the analog amplifiers would have already introduced distortion into the received signal before the echo is canceled by the DSP in the digital domain.
For examples of the conventional art, see V. Aparin, “A New Method of TX Leakage Cancellation in W/CDMA and GPS Receivers”, 2008 RFIC Symposium, RM01D-4. Also refer to U.S. Patent Application Publication No. 2005/0084003 by Duron.
Co-existence of wireless communication links from different wireless standards, and a generally crowded wireless spectrum results in “interfering” radio signals near the frequency of a desired radio signal to be received, as illustrated in FIG. 16.
In an extreme case, the presence of a relatively large interferer near the desired signal makes reception of the desired signal impossible. Even in a relatively good case, the ability to handle a relatively large interferer increases the linearity and baseband filtering requirements of the radio, which in turn increases the radio's cost and power.
One conventional solution to the problem of a large interferer is to increase the linearity and increase the analog baseband requirements of the radio front end. This approach increases both the cost and the power used by the radio.
In another approach illustrated in FIG. 17, the interfering signal is separated at baseband, then up-converted to RF and subtracted from the total RF signal. See Aminghasem Safarian, et al., Integrated Blocker Filtering RF Front Ends, Radio Frequency Integrated Circuits (RFIC) Symposium, Jun. 3-5, 2007, 2007 Institute of Electrical and Electronics Engineers (IEEE), pp. 13-16.